Friday 17 August 2012 15.19 EDT
First published on Friday 17 August 2012 15.19 EDT

For 48 years, Winnie Johnson has held out hope that the Moors murderer, Ian Brady, would reveal the location of her son's body, the last missing victim of Brady and his accomplice, Myra Hindley.

That prospect seemed a tantalising step closer on Friday with news that police are investigating the possibility that Brady has revealed the location of Keith Bennett's grave in a letter he described to his mental health advocate, Jackie Powell.

Yet as police said they are searching for the sealed letter that Powell, whose role is to advise Brady of his rights under the Mental Health Act, said described the location of Bennett's grave, it was unclear whether it even exists.

Powell was arrested on Thursday by Greater Manchester police on suspicion of preventing the lawful burial of a body after she told a Channel 4 documentary about the letter, which she says was addressed to Johnson. The police also searched Brady's cell at the high security hospital at Ashworth in Merseyside, where he is incarcerated. Powell was bailed for three months after documents were seized from her home.

In the Cutting Edge documentary, Powell holds a letter from Brady to her, which she says refers to a second letter written by Brady to Johnson. The programme-makers say they did not see the second letter and were not allowed to read the first, which Powell held while being interviewed.

Martin Bottomley, head of investigative review at Greater Manchester police's major and cold case crime unit, said: "The Moors murders cast a long and dark shadow over the history of our region but in 2009 we reluctantly concluded there was no longer any specific information to identify new search areas and the investigation to find Keith entered a dormant stage. However, we have always stressed this is a case we will never close.

"We have been, and always will be, open to pursuing any new lines of inquiry that arise from significant scientific advances or credible and actionable information."

Bottomley said he did not know if Brady knew the location of the grave or if he was manipulating the public, "but we clearly have a duty to investigate such information on behalf of Keith's family".

Twelve-year-old Bennett was one of the pair's five victims. He disappeared after visiting his grandmother and was killed and buried somewhere on Saddleworth Moor.

Paddy Wivell, producer of the documentary which will be broadcast next week, said he was shocked to hear of Powell's letter and that she intended to return the letter to Brady. He suggested Powell felt a loyalty as she worked with the killer for so long and "apparently she took it back to him".

Powell said she received a letter of instruction and a sealed envelope from Brady via his solicitors. The letter of instruction said the envelope contained three letters – one addressed to Bennett's mother.

The producers urged Powell to tell police about the letter and when she declined they informed the police, who began an investigation on 30 July.

Powell later told the Daily Mirror Brady doesn't want "to take his secrets to the grave", and the letter could afford Johnson, who is in poor health, "the means of her possibly being able to rest".

A quarter of a century ago, Brady returned with police to Saddleworth Moor, along with Hindley, who died in 2002. But police could only identify where Pauline Reade was buried.

Brady was jailed for life in 1966 for the murders of John Kilbride, Lesley Ann Downey and Edward Evans. Hindley was convicted of killing Downey and Evans, and with protecting Brady. She was jailed for life. They finally admitted killing Bennett and Reade in 1987.

David Wilson, a professor of criminology at Birmingham City University and an expert on serial killers, expressed doubt about the existence of a letter. "There is no new information here. Ian Brady went back to the moors in 1987 and said to the police officer: 'Who moves that mountain?' They stopped the investigation then because it's quite clear he didn't have a clue where the body was. This is not about Keith Bennett, or Winnie Johnson, it's about Ian Brady.

"He is a master of trying to inject what is broadcast and printed about him and making sure we interpret the story in the way he would like us to interpret it. He's unique in the sense that most serial killers don't want to talk and communicate. Ian Brady communicates all the time. When you assess what he writes it is usually self-serving nonsense, not remotely reliable."

Brady wants a positive ending, he said, but the killer is "legally insane. So why do we believe he's going to say anything at all that can be reliable?"

Lord Pendry, the former MP for Stalybridge and Hyde, who persuaded Brady to "match his words with deeds" when he revisited the moors in 1987 ostensibly to find Bennett's body, asked: "What's new with the situation? He has been there previously and couldn't find it then." He feared Brady was "leading more people up the garden path."